It is frequently desirable to couple two output devices to temperature sensors in such a way that each output has its own selective temperature setpoint. Each output then switches state when its associated setpoint is reached. Examples of such circuit arrangements may be found in self-cleaning oven controls wherein it is required to prevent the oven door from being opened when the temperature exceeds a determined level, for instance 575.degree. F, while simultaneously controlling the oven heating elements to maintain a cleaning temperature of up to 900.degree. F.
Another example of an application of this invention is in the control of a blower in a domestic forced hot air heating system which is to be turned on when the supply plenum temperature reaches about 130.degree. F. Typically, a high limit safety is provided to shut off the furnance when the temperature exceeds, for instance 180.degree. F.
In both of the above two examples it is desirable to design the circuit so that the failure mode will be safe for as many circuit component malfunctions as possible. It may also be desirable to operate the device directly in a 240 volt, three wire circuit without the use of a stepdown transformer.